The present invention relates generally to a method and apparatus for aligning printing mechanisms and more particularly a method and apparatus for aligning multiple printheads or print cartridges in an ink droplet ejection printer such as a thermal inkjet printer.
One conventional type of printer is one which forms characters and images on a medium, such as paper, by expelling droplets of ink in a controlled fashion so that the droplets land on the medium. Such a printer can be conceptualized as a mechanism for moving and placing the medium in a position such that the ink droplets can be placed on the medium, a printing cartridge which controls the flow of ink and expels droplets of ink to the medium, and appropriate control hardware and software. A conventional print cartridge for an inkjet type printer comprises an ink containment device and a fingernail-sized apparatus, commonly known as a printhead, which heats and expels ink droplets in a controlled fashion. Typically, the printhead is a laminate structure including a semiconductor base, a barrier material structure which is honeycombed with ink flow channels, and an orifice plate which is perforated with holes or orifices with diameters smaller than a human hair and arranged in a pattern which allows ink droplets to be expelled in a controlled pattern. In an inkjet printer the heating and expulsion mechanism consists of a plurality of heater resistors formed in the semiconductor substrate and associated with an ink chamber formed in the barrier layer and one of the orifices in the orifice plate. Each of the heater resistors is connected to the controlling mechanism of the printer such that each of the resistors may be independently energized to quickly vaporize to expel a droplet of ink.
In some applications, more than one inkjet print cartridge will be designed into a printer. Usually this multiple print cartridge assembly is created to accommodate multiple colors of ink. Properly controlling the arrangement of various droplets of ink of different colors will result in a wide spectrum of perceivable colors. The clarity and quality of the resultant image is affected by the accuracy of the placement of the ink droplets on the medium. Printers which use multiple print cartridges to cooperatively form a single image usually require mechanical or electronic adjustment so that ink droplets printed by one cartridge alight at precise locations on the receiving medium relative to those printed by another cartridge in the printer.
Cartridge-to-cartridge alignment has been eliminated in some printers with the use of a single multi-color ink cartridge having a printhead employing three sets of orifices arranged in a group and receiving one color of ink for each group on the printhead. Such a single multi-color print cartridge is inherently self-aligning due to the precise positioning of one set of orifices relative to another on the single orifice plate on the multi-color print cartridge. Even for this cartridge, however, unless other compensation is made, the orifice plate of the printhead should be oriented precisely perpendicular to the direction of travel for accurately printed results.
Mechanical alignment of print cartridges is simple but expensive, requiring precision features created in the orifice plate of the printhead, precision alignment of the cartridges during manufacture to alignment structures or secondary milling of alignment structures or adjustment within the printers cartridge carriage. In each of these foregoing implementations, there are stringent requirements on the printer and the cartridge carriage for either precision during manufacture and long term stability, or complex adjustability and human intervention. Electronic alignment typically requires printing ink droplet dots on a separate region of the medium, scanning the medium with a detector for these dots, then establishing time delays within the printer to compensate for the measured offsets. Again, printer complexity or human intervention and judgment are required to optimize this form of alignment.
Each of the foregoing techniques do not dynamically compensate for movement of the print cartridge within the carriage between alignment cycles due to thermal expansion or wear or loosening within the mechanism. Each of these methods add mechanical or electronic complexity to the printer. Thus, a need exists for a method and apparatus which readily adjusts for horizontal cartridge-to-cartridge alignment errors in a multiple cartridge printer. Furthermore, vertical and rotational offsets also need compensation to precisely align the ink droplets on the media.